What Happens to Atoms During a Chemical Reaction?

Classifying Matter, Chemical Properties, and Chemical Reactions 21 What Happens to Atoms During a Chemical Reaction?

Bo Julian Sandra Gabby The type of atoms The number The number The number of atoms is the same before and and type of atoms are and type of atoms are is the same before and after a chemical reaction, the same before and diferent before and after a chemical reaction, but the number of atoms after a chemical after a chemical but the type of atoms can change. reaction. reaction. can change.

Which friend do you agree with the most? ______Explain why you agree.

______

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Bo Julian Sandra Gabby El tipo de átomos El número de El número y tipo de es el mismo antes y El número y tipo de átomos es el mismo átomos son lo mismo después de una reacción átomos son diferentes antes y después de la antes y después de una química, pero el número antes y después de una reacción química, pero el reacción química. de átomos puede reacción química. tipo de átomos puede cambiar. cambiar.

¿Con qué amigo estás de acuerdo? ______Explica por qué estás de acuerdo.

______

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What Happens to Atoms During a Chemical Reaction? Teacher Notes

Purpose between zinc and hydrochloric acid yields zinc Te purpose of this assessment probe is to chloride and hydrogen gas: Zn + 2HCl elicit students’ ideas about chemical change. ZnCl2 + H2. Te same types of atoms (zinc, Te probe is designed to fnd out whether hydrogen, and chlorine) are present before and students recognize that the same kinds and after the reaction. Te balanced chemical equa- numbers of atoms are present before and after tion shows the same number of atoms before a chemical reaction. the reaction as after the reaction (one atom of zinc, two atoms of hydrogen, two atoms Type of Probe of chlorine) even though the zinc combined Concept cartoon with chlorine and hydrogen separated from the hydrochloric acid. Te law of conservation Related Concepts of matter supports the idea that the number Atom, balanced equation, chemical change, and type of atoms stay the same because no chemical reaction, conservation of matter new atoms are created or destroyed. Explanation Administering the Probe Te best answer is Julian’s: “Te number and Tis probe is best used with students in grades type of atoms are the same before and after 6–12 once students can use the idea of atoms the chemical reaction.” During a chemical to support their explanation. Te probe can be reaction, atoms are rearranged to form diferent extended by having students draw a model or substances but the total number of atoms and provide a symbolic representation, such as a the kinds of atoms are the same before and chemical equation, to support their thinking. after the reaction. For example, the reaction

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Related Disciplinary Core Ideas Related Research and Crosscutting Concepts • Andersson (1991) investigated children’s From the Framework (NRC notions of chemical change and found 2012) they appear to fall into six categories: (1) it just happens; (2) matter just disappears; 6–8 PS1.A: Structure and Properties of (3) the product materials must have been Matter inside the starting materials; (4) the prod- • Substances are made from diferent types uct material is just a modifed form of the of atoms, which combine with one another starting material; (5) the starting material in various ways. Atoms form molecules just turns into the product material; and that range in size from two to thousands (6) the starting materials interact to form of atoms. the product materials. 6–8 PS1.B: Chemical Reactions • In general, students have difculty develop- • Substances react chemically in characteris- ing an adequate conception of the chemical tic ways. In a chemical process, the atoms combination of elements until they can that make up the original substances are interpret combination at the molecular regrouped into diferent molecules, and these level (Driver et al. 1994). new substances have diferent properties • In a study of 100 high school students who from those of the reactants. completed a unit on chemical change, most • Te total number of each type of atom students failed to use atoms and molecules is conserved, and thus the mass does not in their explanations even though they had change. been emphasized in their chemistry course. 6–8 Crosscutting Concept: Energy and Some students also listed heat, cold, or Matter decay as reactants or products. Although • Matter is conserved because atoms are con- most students in the classes could state served in physical and chemical processes. the defnition of a chemical reaction and 9–12 PS1.B: Chemical Reactions balance chemical equations, they still had • Chemical processes, their rates, and whether difficulty describing chemical changes or not energy is stored or released can be (Hesse and Anderson 1992). understood in terms of the collisions of • Ben-Zvi, Eylon, and Silberstein (1982) molecules and the rearrangement of atoms found that students have great difculty into new molecules. changing their thinking when they are asked • Te fact that atoms are conserved, together to transition from observable changes in with knowledge of the chemical properties substances to the atomic molecular level of the elements involved, can be used to to explain observable changes in terms of describe and predict chemical reactions. the interactions between individual atoms 9–12 Crosscutting Concept: Energy and and molecules. Matter • To master chemistry, students need to • Te total amount of energy and matter in develop an understanding of chemical closed systems is conserved. ideas at three diferent levels: macroscopic, particle, and symbolic (Gabel 1999).

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Suggestions for Instruction and and that this is an example of another way Assessment that the atomic/molecular theory can be • Te idea that a chemical reaction involves used to explain a wide variety of matter the recombination of atoms is not easily phenomena. understood by students. In addition, the • Researchers suggest that, if students are to rules for writing and balancing chemical acquire the scientifc conception of chem- equations may seem relatively simple and ical change, both teachers and curriculum straightforward, yet the concept of balancing developers must begin to anticipate the a chemical equation is not easily understood deeper misconceptions that afect students’ by students who have strongly held alternative thinking about chemical change (Hesse ideas. Terefore, it is imperative to surface and Anderson 1992). the strongly held ideas in this probe and • Have students create two diferent models help students work through them using to show a chemical reaction and use both models and other conceptual techniques models to explain what happens to the to give up their preconceptions and move molecules. For example, they can create a them toward a chemical understanding of physical clay model with atoms (and their chemical reactions. molecules) made of small balls of clay and a • Students who recognize the number of atoms symbolic representation (balanced chemical stays the same but may think the type of reaction). Research has shown that when atoms changes during a chemical reaction students create and compare diferent models need to develop a key idea that while an for the same phenomena their conceptual atom retains its chemical identity during understanding of the chemical idea increases a chemical reaction, a molecule does not. (Harrison and Treagust 1996). • Ball and stick models can be used to show how substances break apart and recombine References to form new substances with the same Andersson, B. 1991. Pupils’ conception of matter type and number of atoms (but diferent and its transformations (age 12–16). Studies molecules), but be sure to point out the in Science Education 18: 53–85. limitations of using these models so they Ben-Zvi, R., Eylon, B., & Silberstein, J. 1982. are not interpreted literally. Students vs. chemistry: A study of student • Students should see a great many examples conceptions of structure and process. Paper of reactions between substances that pro- presented at the annual conference of the duce new substances that are very diferent National Association for Research in Science from the original reactants to understand Teaching. Fontana, WI. that the types of atoms are the same but Driver, R., A. Squires, P. Rushworth, and V. the properties of the new substances are Wood-Robinson. 1994. Making sense of secondary diferent. Start of with examples of familiar science: Research into children’s ideas. New York: reactions such as burning sugar, adding RoutledgeFalmer. baking soda to vinegar, and rusting to Gabel, D. 1999. Improving teaching and learning determine whether new substances are through chemistry education research: A look formed, and compare the number and to the future. Journal of Chemical Education type of atoms before and after the reaction. 76 (4): 548–554. • Help students see that the idea of atoms Harrison, A., and D. Treagust. 1996. Secondary can be used to explain chemical reactions students’ mental models of atoms and molecules:

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Implications for teaching chemistry. Science National Research Council (NRC). 2012. A Education 80 (5), 509–534. framework for K–12 science education: Practices, Hesse, J., and C. Anderson. 1992. Students’ con- crosscutting concepts, and core ideas. Washington, ception of chemical change. Journal of Research DC: National Academies Press. in Science Teaching 29 (3): 277–299.

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